Strand heating of wire, wire rod, sheet or strip (hereinafter collectively "wire and strip") is an operation commonly performed as an intermediate step prior to other treatments such as oxide coating, galvanizing, quenching or further drawing or rolling. These subsequent operations are often performed in line with strand heating. Processes such as oxide coating, galvanizing and subsequent drawing or rolling require that the surface of the wire be clean so that coatings such as oxide, zinc and lubricant will adhere to the steel wire or strip.
Wire or Wire Rod Processing
For high carbon wire, it is also important that the strand does not decarburize the wire. In most existing strand heating processes, the exiting wire is dirty and has an oxidized surface, thus requiring in-line pickling prior to further processing. Because the costs attributed to the pickling process have risen substantially due to environmental considerations, an economical process for strand annealing with a clean, oxide-free and decarb-free surface is desired.
There are currently a number of methods for annealing wires. However, all of the current methods for annealing wires contain certain disadvantages.
Generally, the present state of the art provides for the strand annealing of wires or wire rods either with or without the subsequent requirement for in-line pickling.
Strand annealing with in-line lead bath is the traditional strand annealing process that is still currently in wide practice. The heating rate is very fast and good temperature uniformity is achieved at high production rates. As a result, this particular process is economical. The drawback in this process, however, is the need to avoid excessive lead typically being dragged out from the bath. The wire surface must not be wetted by the molten lead, and the wire surface must be kept in a dirty state (containing oxide and lubricant residues). The drawback to the use of this process is the need to clean the wire, usually by an in-line pickling process, prior to undergoing further processing. Because of the problems with lead contamination, this process poses a necessary but environmentally unfriendly procedure to prevent any such contamination, which increases the expenses necessary for the strand annealing process.
Another process for strand annealing which requires in-line pickling is strand annealing in fluidized beds. In this process, continuous fluid bed lines were developed for annealing strand wires. The beds are fluidized and heated using products of combustion, typically with natural gas and air. The heating rates are substantially lower than lead baths, so the bed lengths are greater and production rates are lower. It is possible to achieve higher production rates by preheating the wire via induction heating of up to about 1300.degree. F. Since the atmosphere in the fluidized bed contains the products of combustion (i.e., nitrogen, carbon dioxide and water), the wires will be oxidized and the surface will be decarburized. As a result, in-line pickling is required before further processing is performed in this process.
A variety of processes for producing clean wires that do not require in-line pickling are also known in the art. One such process is strand annealing in multi-tube furnaces. In this process, the wires are heated in individual tubes, typically in a pure hydrogen atmosphere. Up to sixteen tubes, each containing a wire, are mounted in a large furnace. Even in the pure hydrogen atmosphere, the production rates are slow due to the poor heat transfer between gas and metal. The tube length can not be made much longer than about 40 feet due to excessive drag on the wire. This process produces bright (oxide-free) wires and the wires do not require in-line pickling prior to further processing. This method, however, is not economical for large-scale production.
Yet another process for producing clean wires without in-line pickling is heating the fluidized bed indirectly by inserting a set of firing tubes in the bed. The bed is fluidized with an inert gas like nitrogen. This process produces bright, decarb-free wire. This process is disadvantageous because it requires a large amount of nitrogen, making the process uneconomical, even when the nitrogen is recycled.
Therefore, it is an object of the present invention to provide for a process for strand annealing wires that does not require pickling prior to further processing. Additionally, it is an object of this invention to provide a wire strand annealing process that does not require a large amount of either nitrogen or hydrogen such that the process becomes uneconomical.
Sheet or Strip Processing
Steel sheet and strip are usually produced by hot rolling from slabs in continuous hot-strip mills. For the low-carbon steels generally used for sheet and strip, the finishing temperature at the last rolling stand is generally about 1550.degree. F. Since the rolling mills operate in air, the steel sheet or strip undergoes oxidation. The presence of an oxide scale on the sheet or strip is objectionable if the sheet or strip is to be processed further. For example, if the sheet is for drawing applications, removal of the oxide is essential since its presence results in short die life and poor surface finish on the finished product. Oxide removal is also essential if further processing involves any type of coating such as metallic coatings (e.g. zinc) or non-metallic coatings (e.g. paint).
A common method of cleaning the sheet or strip after hot rolling is by passing through the continuous pickling lines. In these pickling lines, the steel sheet or strip passes through a series of baths based on either sulfuric acid or hydrochloric acid. The environmental disadvantages associated with acid pickling lines are well known and high capital expense is incurred for the treatment of waste pickle liquors and prevention of corrosion of equipment and buildings. Another method known in the art for removing scale from sheet or strip is by continuously blasting the sheet or strip with abrasive grit or steel shot, thereby mechanically removing the scale. While these shot-blasting methods avoid the problems of waste pickle-liquor disposal, the process is inherently slow and therefore uneconomical.
It is an object of this invention to provide a method for removing scale from steel sheet or strip products in an environmental friendly manner and operating at temperatures at or above 1200.degree. F.